Your better off just changing the hoses to braided and if changing the discs get drilled and grooved. Going bigger 5L for the sake of it isn't worth it.... unless your 4L gets bigger
It's a bit of a balls ache on the rears too cos you have to change the bearing carriers too, cos the bolt holes for mounting the 5l calipers are spaced at a larger radius from the axle centre than on the 4l cars.